518 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 34, NO. 3, JUNE 2006

Operational Characteristics of a 14-W 140-GHzGyrotron for Dynamic Nuclear Polarization

Colin D. Joye, Student Member, IEEE, Robert G. Griffin, Melissa K. Hornstein, Member, IEEE,Kan-Nian Hu, Kenneth E. Kreischer, Member, IEEE, Melanie Rosay, Michael A. Shapiro, Member, IEEE,

Jagadishwar R. Sirigiri, Member, IEEE, Richard J. Temkin, Fellow, IEEE, and Paul P. Woskov, Senior Member, IEEE

Abstract—The operating characteristics of a 140-GHz 14-W longpulse gyrotron are presented. The device is being used in dynamicnuclear polarization enhanced nuclear magnetic resonance (DNP/NMR) spectroscopy experiments. The gyrotron yields 14 W peakpower at 139.65 GHz from the TE(0,3) operating mode using a12.3-kV 25-mA electron beam. Additionally, up to 12 W peak hasbeen observed in the TE(2,3) mode at 136.90 GHz. A series of modeconverters transform the TE(0,3) operating mode to the TE(1,1)mode. Experimental results are compared with nonlinear simula-tions and show reasonable agreement. The millimeter-wave outputbeam was imaged in a single shot using a pyroelectric camera.The mode patterns matched reasonably well to theory for both theTE(0,1) mode and the TE(1,1) mode. Repeatable mode patternswere obtained at intervals ranging from 0.8 s apart to 11 min apartat the output of the final mode converter.

Index Terms—Dynamic nuclear polarization, gyrotron, mil-limeter wave.

I. INTRODUCTION

A140-GHz 14-W-long pulse gyrotron is in use for dy-namic nuclear polarization (DNP) enhanced nuclear

magnetic resonance (NMR) spectroscopy experiments in theFrancis Bitter Magnet Laboratory at the Massachusetts Instituteof Technology (MIT), Cambridge. In order to increase thesignal-to-noise ratio (SNR) of the spectrum, the sample ispolarized by millimeter-wave radiation (MMW) introducedinto the DNP probe near the electron Larmor frequency and byradio frequency radiation near the nuclear Larmor frequencies.SNR enhancement factors on the order of several hundred arepossible using this technique [1], [2]. A number of modestpower, high-frequency gyrotrons have been built for variousDNP applications recently [3]–[5].

A gyrotron is a vacuum electron device (VED) employing astrong magnetic field, an electron beam, and an electromagnet-

Manuscript received September 1, 2005; revised November 28, 2005.This work was supported by National Institutes of Health (NIH), NationalInstitute for Biomedical Imaging and Bioengineering (NIBIB) under ContractEB001965, Contract EB002804, and Contract EB002026.

C. D. Joye, M. A. Shapiro, J. R. Sirigiri, R. J. Temkin, and P. P. Woskov arewith the Plasma Science and Fusion Center, Massachusetts Institute of Tech-nology, Cambridge, MA 02139 USA (e-mail: cjoye@mit.edu).

R. G. Griffin and K.-N. Hu are with the Department of Chemistry and theFrancis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cam-bridge, MA 02139 USA.

M. K. Hornstein is with the Naval Research Laboratory, Washington, DC20375 USA.

K. E. Kreischer is with the Northrop Grumman Corporation, Electronics Sys-tems, Rolling Meadows, IL 60008 USA.

M. Rosay is with the Bruker BioSpin Corporation, Billerica, MA 01821 USA.Digital Object Identifier 10.1109/TPS.2006.875776

ically resonant cavity. Detailed descriptions of the physics andengineering features of the gyrotron may be found in severalrecent books [6]–[8]. The electrons gyrate around the magneticfield lines at the electron cyclotron frequency in a helical motionas they traverse through the tube structure. Inside the resonator,a fast wave interaction between the electron beam and MMWtakes place which converts some of the transverse kinetic energyof the electron beam into MMW energy in the form ofelectromagnetic modes. The transverse dimensions of the res-onator can be of the order of several wavelengths. This enablesthe generation of high average power at high frequencies in gy-rotrons due to lower thermal losses in the resonator walls whencompared to conventional slow wave devices, such as travelingwave tubes and backward wave oscillators.

This gyrotron is based on an older design that was devel-oped and used for plasma diagnostics [9]. That gyrotron wasmodified and used successfully for DNP/NMR research, and isdescribed in detail in [10]. The gyrotron used a high-voltageelectron gun, which operated at 40 to 65 kV, requiring cumber-some oil cooling. The gun was also a triode configuration witha cathode, modulation anode, and a ground anode. The present140-GHz gyrotron system contains a new diode electron gun,which operates at a low voltage of 12.3 kV at up to 25 mA.Output pulses up to 15 s long at 50% duty cycle are possible.For single-pulse operation, an emission period as long as 5 minis easy to achieve. The lower voltage has resulted in a much sim-pler power supply system.

This paper reports the operating characteristics of the present140-GHz 14-W gyrotron. The presentation of this material isorganized as follows: First, a brief overview of the gyrotron ispresented, followed by the results of the experiments performedto map the operating characteristics which include a mode map,power and frequency measurements, simulations, and pyroelec-tric camera images of the mode patterns. In the final section,concluding remarks are given.

II. OVERVIEW

The system block diagram is shown in Fig. 1. The hollow,annular electron beam is emitted from a thin, indirectly heatedring cathode of a magnetron injection gun (MIG) at one extremeof the tube. This beam is guided and compressed through thecavity circuit by a solenoidal magnetic field with a peak valueof approximately 5.08 T centered in the interaction region. Thepotential between the cathode and anode surfaces of the MIG is

12.3 kV and the current can be controlled from about 10 to 30mA by adjusting the alternating current (ac) heater filament cur-rent. The magnetic field at the cathode is fine tuned by means of

0093-3813/$20.00 © 2006 IEEE

JOYE et al.: OPERATIONAL CHARACTERISTICS OF A 14-W 140-GHZ GYROTRON FOR DYNAMIC NUCLEAR POLARIZATION 519

Fig. 1. Gyrotron block diagram showing the electron gun (MIG), main magnet,cavity, collector, internal mode converters, and quartz window.

TABLE IOPERATING PARAMETERS

a water cooled copper coil bolted to the outside of the supercon-ducting magnet Dewar, which is capable of altering the cathodefield by about 0.2 T. The vacuum base pressure is on the

torr scale and the operating pressure is kept well belowtorr. A summary of the operating parameters for this gy-

rotron is presented in Table I.The present gyrotron tube differs from the original tube

mostly in the region of the electron gun. Most of the parts ofthe present electron gun were built by MIT and assembled byMIT, with exception of the cathode stalk, which was built bySemicon Associates (Lexington, KY). The previous design ofthe tube had very inefficient pumping with a small constriction,three 90 bends and nearly a meter of vacuum lines fromthe emitter region to a 40 L/s vacuum pump. In addition, thepumping configuration prevented proper baking of the entiregyrotron tube under vacuum. The present gun was designedto allow for direct pumping of the electron gun via four slots(0.74 cm 4.22 cm) cut through the cathode stalk for efficientpumping of the emitter (Fig. 2). The cathode stalk alignment iscontrolled by shims between the two flanges at the base of thegun. The shims limit the depth of the flange cut into the coppergasket. The ac heater is insulated from the high voltage with aceramic feed-through while two large ceramic breaks insulatethe anode and the 8 L/s vacuum ion pump. The electron gunvacuum pump is located about 23 cm in a straight line from theemitter. The present design also allows for possible removalof the gyrotron tube from the superconducting magnet withoutdisassembly (except for removal of the ion pump magnets).The tube can withstand being baked out at up to 200 .

1http://www.semiconassociates.com

Fig. 2. Diagram of the current electron gun showing the location of thepumping slots and vacuum pump.

Fig. 3. Schematic of the resonator showing (a) the downtaper, (b) cavitystraight section, and (c) uptaper section along with (d) the simulated electricfield profile of the TE mode in arbitrary units.

The electron gun was designed with a beam pitch factor ofapproximately 1.6, a beam radius of 1.86 mm (corresponding tothe second radial maximum of the mode), and transverseoptical velocity spread of 3.2% from simulations at the oper-ating point using EGUN [11]. The calculated total transversevelocity spread including surface roughness and thermal effects[12] is approximately 6%. The actual beam pitch factor duringoperation is very sensitive to the magnetic field at the cathode.

After the electron beam passes through the beam tunnel, itenters the resonator section of the cavity (Fig. 3). On the end ofthe cavity through which the electron beam enters, the resonatorradius is downtapered toward the electron gun at an angle of 1such that it is cut off to the operating MMW fields in the res-onator in order to prevent power from leaking out. The gyrotroncavity resonator is a circular waveguide straight section 28.25mm long (approximately 13 wavelengths) with a radius of 3.48mm. In this section, the cyclotron resonance maser (CRM) insta-bility causes some of the perpendicular momentum in the elec-tron beam to be converted into MMW energy near the cyclotronfrequency of the electrons. At the end of the straight section, thecavity radius is uptapered with a 2 angle. The uptaper allowsthis MMW radiation to diffract out of the cavity and is designedto prevent further interaction of the MMW and electron beam.The theoretical diffractive of the cavity is almost 7000. Thetheoretical ohmic is approximately 28 000, but we estimate amuch smaller value of about 14 000. The calculated temperaturerise in the cavity during CW operation is approximately 0.2 K.

The cavity is water cooled to prevent it from becoming de-tuned due to thermal expansion. The heat lost by the finite elec-trical conductivity of the copper is carried away by a water-cooled jacket brazed to the cavity.

520 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 34, NO. 3, JUNE 2006

After the spent electron beam exits the cavity circuit, it iscollected at the other extreme of the tube by a water-cooledcollector.

The MMW radiation travels through the uptaper and collectorsections and then propagates through a to axisym-metric rippled wall mode converter of beat periods fol-lowed by a to mode converter that also usesperiods [9]. The efficiencies of each of these mode convertershas been measured to over 99% [10]. A mirror and miter jointat the end of the second mode converter prevent stray electronsfrom striking the vacuum window and carry the modeout of the gyrotron via a 1.27-cm-diameter overmoded copperwaveguide. The fused quartz window is 2.73 cm in diameter and3.43 mm thick and is designed to be transparent to MMW at 140GHz. An all-metal valve was added close to the quartz windowfor connecting a turbo pump during bake-out.

Outside of the gyrotron, the mode is converted to themode using a serpentine rippled wall mode converter [13].

This mode converterwas designedusing coupled modeequationsfor a circular waveguide bent in a circular arc where one beatperiod is made up of two bent 6.53 cm long, 1.27 cm diametercopper sections with a bending radius of 1.21 m placed end toend.Thestructureconsistsofsevensuchperiodsyieldingadesignefficiency ofover 94%at 140 GHz [14]. The structure was createdby heating a copper pipe packed with sand and forcing it into acarved aluminum block to create the serpentine shape.

After this mode converter, the power is finally sent to theDNPprobebymeansofanovermodedcylindrical1.27-cm-diam-eter copper waveguide. The losses in this waveguide total approx-imately 7 dB and the bulk of these losses appear in a downtapersection just before entry to the DNP probe [10], [15], [16].

III. EXPERIMENTAL RESULTS

A harmonic heterodyne receiver system was used to measurethe frequency of the gyrotron output. The receiver employs alow frequency local oscillator (LO) of between 8–18 GHz anda harmonic mixer to mix the gyrotron signal with a harmonicof the LO (typically the tenth harmonic). The intermediate fre-quency (IF) is limited to 150–500 MHz by bandpass filters andis amplified by a series of solid state amplifiers. The result isfast fourier analyzed on an oscilloscope. The LO frequency iscounted by a frequency counter, multiplied by the harmonicnumber, and added to or subtracted from the IF to determinethe gyrotron frequency. The system can accurately measure thegyrotron frequency to within 1 MHz or better.

The output power was measured using a 4-in analog Peltierjunction dry calorimeter from Scientech Inc. We modifiedthis calorimeter by applying a thin (approximately 0.3 mm)absorbing layer of 3M Nextel paint to enhance the absorption ofMMW [17]. The power absorption has recently been measuredat 140 GHz to be 82% using an E8363B Agilent Precision Net-work Analyzer (PNA) with Olsen Microwave Lab millimeterwave extenders that work in the WR-8 (90–140 GHz) band.

A. Mode Map

The mode map for this gyrotron was generated by holdingthe operating voltage fixed at 12.3 kV while scanning the main

Fig. 4. Mode map for the 140-GHz gyrotron. Heavy circles indicate the loca-tion of highest observed power in each mode.

TABLE IIMODE TABLE

magnetic field and the gun coil magnetic field (Fig. 4). The fila-ment current was adjusted to keep the beam current at a nominalvalue of around 25 mA. The gyrotron was set to a duty cycle of50% and a pulse length of 1 s.

Table II lists the different modes and frequencies observedin generating this mode map. The measured frequencies werematched to theory through a cold cavity code originally devel-oped by Fliflet and Read [18]. In this cold cavity code, the ra-dius of the resonator had to be decreased from its theoreticalvalue of 3.480 to 3.478 mm to match the measured frequencyof the operating mode. The change in radius is consis-tent with the manufacturing tolerance of the cavity. The otherfrequencies were then consistent with the measured data. Theaxial profiles resulting from the cold cavity code indicated theaxial mode index, while the resonant frequency and diffractive

are output values from the code.A transient frequency shift of approximately 0.2 MHz during

the first 3 s of the pulse at 12.3 kV has been observed due to thethermal expansion of the cavity. The sensitivity of frequencyshift due to thermal expansion was found to be 2.2 MHz/K.Additionally, there are frequency fluctuations due to the powersupply regulation on the order of 0.2 MHz. The gyrotron’s fre-quency fluctuation sensitivity due to voltage ripple has been de-termined to be 0.14 ppm/V [10], [19].

The interaction was modeled using a nonlinear time dependentsimulation code MAGY, developed jointly at the University ofMaryland and the Naval Research Laboratory [20]. The MAGYsimulation is compared to the measured power from this cavitydesign in the mode in Fig. 5 over a range of main magneticfield. Using a calculated transverse velocity spread of 6% for the

JOYE et al.: OPERATIONAL CHARACTERISTICS OF A 14-W 140-GHZ GYROTRON FOR DYNAMIC NUCLEAR POLARIZATION 521

Fig. 5. Power output for the TE mode versus main magnetic field com-paring the measurement to simulation.

simulation and a conductivity of half that of ideal copper for thecavity walls, a beam pitch factor of 1.45 resulted in a good fit tothe measured data. Other operating parameters were the same asin Table I. The actual value of beam pitch factor in experiment isvery sensitive to the magnetic field at the cathode, and it appearsthat the low beam pitch factor, lower than the design value of 1.6,is limiting the output power.

The theoretical minimum oscillation start current for theoperating mode was calculated to be 10 mA [21] at 12.3

kV, 5.08 T, and a total factor of approximately 5000. Thissimulated value matched the experimentally observed valueunder the same conditions.

B. Millimeter-Wave Imaging

The MMW output of the gyrotron was imaged using thePyrocam III pyroelectric camera detector from Spiricon, Inc.[22]. This camera consists of an array of 124-by-124 calibrated

pyroelectric sensors with a spacing of 100 m betweeneach pixel. There is a motorized chopper blade over the sensorarray which can operate at 48 or 24 Hz, or can be externallytriggered for pulse operation. The chopper is necessary for CWbeams because the pyroelectric crystals are only sensitive tochanges in signal. Although this camera was designed for laserapplications for wavelengths in the Terahertz range and wastested at frequencies as low as 0.2 THz (1.5 mm wavelength)[23], we have found that it also works very well at microwavefrequencies where the wavelength is above 2 mm, providedthe waveguide aperture is small enough and the maximumpower rating for the sensor is not exceeded. To our knowledge,this is the first time that a millimeter wave beam of such longwavelength has been directly imaged with a high resolution andhigh speed camera. Similar measurements have also been madeof a 250 GHz (1.2-mm wavelength) gyrotron beam at MIT byBajaj et al. [24].

The pyroelectric camera provides the operator the advantageof viewing the mode pattern in real time and can thus beused to follow the beam shape over time in increments assmall as tens of milliseconds. This may be advantageous forsystems where the power output is not stable over time. Itcan also be set up at various points in a beam system as anaid in alignment and measurement. The pyroelectric cameraratings are listed in Table III.

TABLE IIIPYROELECTRIC CAMERA RATINGS AT 24 HZ [22]

Fig. 6. (a) Measured gyrotron output. (b) Theoretical TE mode. (c) Mea-sured final mode converter output. (d) Theoretical TE mode. Scale is nor-malized linear power.

Two measurements were made using the camera: one at thegyrotron output window and the other at the end of theto mode converter. In both cases, the pyroelectric camerawas placed as close to the output as the chopper would allow,with the sensor less than three wavelengths away from theoutput aperture in both cases.

The measured and calculated mode patterns are shown inFig. 6(a) and (b), respectively, near the quartz window wherethe pattern should match the mode, and in Fig. 6(c) and(d) after the final mode converter where the pattern should matchthe mode. The experimental patterns were captured at lowpower (approximately 0.4 W average) to avoid damage to thepyroelectric camera. While there is evidence of mode mixtures,the measured mode patterns matched well enough to the theo-retical patterns for our present application.

Fig. 7(a) and (b) shows two adjacent images of the final modeconverter output captured 0.8 s apart. Fig. 7(c) and (d) showsthe same mode captured 11 min apart. These images show theexcellent repeatability of the gyrotron pulses and the capabilityof the camera to monitor the long term stability of the gyrotronoutput beam.

IV. CONCLUSION

A 140-GHz 14-W long pulse DNP gyrotron has been oper-ationally characterized. A mode map of the operating regimesof the device has been generated and shows 14 W peak powerfrom a 12.3-kV, 25-mA electron beam with a radius of approx-imately 1.86 mm and an estimated perpendicular momentum

522 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 34, NO. 3, JUNE 2006

Fig. 7. Mode measurements at the output of the final mode converter (a)–(b)0.8 s apart and (c)–(d) 11 min apart. Scale is normalized linear power.

spread of 6% for the operating mode at 139.65 GHz. Thedesign value of the beam pitch factor was approximately 1.6,but we have found that a value of 1.45 in simulation fits themeasured data well. Additionally, up to 12 W peak power hasbeen observed in the mode at 136.90 GHz. The measuredstart current values agree well with theory. The nonlinear codeMAGY has been used to analyze the output power with goodagreement assuming a lower value of beam pitch factor in theelectron beam.

For the first time on the 140-GHz gyrotron system, a pyro-electric camera has been used to image the entire mode pat-tern in a single capture. Repeatable images were captured overa wide range of time scales. The measured mode patterns forthe 140-GHz gyrotron matched reasonably well for both the

mode and the mode. The pyroelectric camera is apromising technology that offers wide dynamic range, high sen-sitivity and real time image capture at up to several frames persecond. These features make it useful for such tasks as aligningwaveguide and quasi-optical systems, and for long term moni-toring of the output beam.

ACKNOWLEDGMENT

The authors would like to thank J. Bryant for his assistancewith the gyrotron measurements.

REFERENCES

[1] R. A. Wind et al., “Applications of dynamic nuclear polarization in13C NMR in solids,” Progress NMR Spectroscopy, vol. 17, pp. 33–67,1985.

[2] L. R. Becerra, G. J. Gerfin, R. J. Temkin, D. J. Singel, and R. G. Griffin,“Dynamic nuclear polarization with a cyclotron resonance maser at 5Tesla,” Phys. Rev. Lett., vol. 117, pp. 3561–3564, 1993.

[3] K. E. Kreischer, C. Farrar, R. Griffin, and R. Temkin, “The use of a 250GHz gyrotron in a DNP/NMR spectrometer,” in Proc. 23rd Int. Conf.Infrared Millim. Waves, 1998, pp. 341–357.

[4] T. Idehara, I. Ogawa, S. Mitsudo, M. Pereyaslavets, N. Nishida, andK. Yoshida, “Development of a frequency tunable, medium power gy-rotron (gyrotron FU series) as submillimeter wave radiation sources,”IEEE Trans. Plasma Sci., vol. 27, no. 2, pp. 340–354, Apr. 1999.

[5] M. K. Hornstein, V. S. Bajaj, R. G. Griffin, K. E. Kreischer, I.Mastovsky, M. A. Shapiro, J. R. Sirigiri, and R. J. Temkin, “Secondharmonic operation at 460 GHz and broadband continuous frequencytuning of a gyrotron oscillator,” IEEE Trans. Electron Devices, vol.52, no. 5, pp. 798–807, May 2005.

[6] R. J. Barker, N. C. Luhmann, J. H. Booske, and G. S. Nusi-novich, Modern Microwave and Millimeter-Wave Power Elec-tronics. Hoboken, NJ: Wiley-IEEE, 2005.

[7] G. S. Nusinovich, Introduction to the Physics of Gyrotrons. Balti-more, MD: Johns Hopkins Univ. Press, 2004.

[8] M. V. Kartikeyan, E. Borie, and M. K. A. Thumm, Gyrotrons. NewYork: Springer–Verlag, 2004.

[9] J. S. Machuzak, P. Woskoboinikow, W. J. Mulligan, D. R. Cohn, M.Gerver, W. Guss, M. Mauel, R. S. Post, and R. J. Temkin, “137-GHzgyrotron diagnostic for instability studies in Tara,” Rev. Sci. Instrum.,vol. 57, no. 8, pp. 1983–1985, 1986.

[10] L. R. Becerra, G. J. Gerfin, B. F. Bellew, J. A. Bryant, D. A. Hall,S. J. Inati, R. T. Weber, S. Un, T. F. Prisner, A. E. McDermott, K.W. Fishbein, K. E. Kreischer, R. J. Temkin, D. J. Singel, and R. G.Griffin, “A spectrometer for dynamic nuclear polarization and electronparamagnetic resonance at high frequencies,” J. Mag. Reson., vol. 117,pp. 28–40, 1995.

[11] W. B. Hermannsfeldt, EGUN—An electron optics and gun designprogram Stanford Linear Accelerator Center, Tech. Rep. SLAC-331,UC-28, Oct. 1988.

[12] J. M. Baird and W. Lawson, “Magnetron injection gun (MIG) designfor gyrotron applications,” Int. J. Electron., vol. 61, no. 6, pp. 953–967,1986.

[13] C. Moeller, “Mode converters used in the Doublet III ECH microwavesystem,” Int. J. Electron., vol. 53, pp. 587–593, 1982.

[14] M. Hornstein, “Design of a 460 GHz second harmonic gyrotron oscil-lator for use in dynamic nuclear polarization,” M.S. thesis, Dept. Elec.Eng. Comp. Sci., Massachusetts Institute of Technology, Cambridge,2001.

[15] M. Rosay, “Sensitivity enhanced nuclear magnetic resonance in biolog-ical solids,” Ph.D. dissertation, Dept. Chem., Massachusetts Institute ofTechnology, Cambridge, 2001.

[16] M. Hornstein, “A continuous wave second harmonic gyrotron oscil-lator at 460 GHz,” Ph.D. dissertation, Dept. Elec. Eng. Comp. Sci.,Massachusetts Institute of Technology, Cambridge, 2005.

[17] K. E. Kreischer, J. B. Schutkeker, B. G. Danly, W. J. Mulligan, and R.J. Temkin, “High efficiency operation of a 140 GHz pulsed gyrotron,”Int. J. Electron., vol. 57, no. 6, pp. 835–850, 1984.

[18] A. W. Fliflet and M. E. Read, “Use of weakly irregular waveguidetheory to calculate eigenfrequencies, Q values and RF field functionsfor gyrotron oscillators,” I. J. Electron., vol. 51, pp. 475–484, 1981.

[19] D. A. Hall, “Dynamic nuclear polarization of biological systems at highmagnetic fields,” Ph.D. dissertation, Massachusetts Institute of Tech-nology, Cambridge, 1998.

[20] M. Botton and T. M. Antonsen, Jr., “MAGY: a time-dependent code forsimulation of slow and fast microwave sources,” IEEE Trans. PlasmaSci., vol. 26, no. 3, pp. 882–892, Jun. 1998.

[21] B. G. Danly and R. J. Temkin, “Generalized nonlinear harmonic gy-rotron theory,” Phys. Fluids, vol. 29, no. 2, pp. 561–567, 1985.

[22] Pyrocam III, Model PY-III-C-B Spiricon, Inc.. Logan, UT, 2001.[23] Pyrocam III, New Product Announcement Spiricon, Inc.. Logan, UT,

Jan. 2003.[24] V. S. Bajaj et al., “A Long-Term, Stable, CW 250 {GHz} Gyrotron,” ,

unpublished.

Colin D. Joye (S’03) received the B.S. degree inelectrical engineering and computer science fromVillanova University, Villanova, PA, in 2002, and theM.S. degree in electrical engineering and computerscience, in 2004, from Massachusetts Institute ofTechnology (MIT), Cambridge, where he is currentlyworking toward the Ph.D. degree.

In 2002, he joined the Waves and Beams Divisionat the Plasma Science and Fusion Center (PSFC),MIT as a Research Assistant. His current researchinterests include gyrotron oscillator and amplifier

sources at the millimeter and submillimeter wavelengths.

JOYE et al.: OPERATIONAL CHARACTERISTICS OF A 14-W 140-GHZ GYROTRON FOR DYNAMIC NUCLEAR POLARIZATION 523

Robert G. Griffin received the B.S. degree in chem-istry from the University of Arkansas, Fayetteville, in1964 and the Ph.D. degree in physical chemistry fromWashington University, St. Louis, MO, in 1969. Hedid his postdoctoral research in physical chemistryat the Massachusetts Institute of Technology (MIT),Cambridge, with Prof. J. S. Waugh.

In 1972, after completing his postdoctoral training,he assumed a staff position at the Francis Bitter Na-tional Magnet Laboratory (FBML), MIT. In 1984,he was promoted to Senior Research Scientist, and

was appointed to the faculty in the Department of Chemistry, MIT, in 1989.In 1992, he became Director of the FBML and is concurrently Director of theMIT-Harvard Center for Magnetic Resonance, where he had been Associate Di-rector since 1989. He has published more than 300 articles concerned with mag-netic resonance methodology and applications of magnetic resonance (NMRand EPR) to studies of the structure and function of a variety of chemical, phys-ical and biological systems. In the last decade, this research has focused on thedevelopment of methods to perform structural studies of membrane and amy-loid proteins and on the utilization of high-frequency (>100 GHz) microwavesin EPR experiments and in the development of DNP/NMR experiments at thesefrequencies. He has served on numerous advisory and review panels for the Na-tional Science Foundation and the National Institutes of Health.

Melissa K. Hornstein (S’97–M’05) received the B.S. degree in electrical andcomputer engineering from Rutgers University, New Brunswick, NJ, in 1999,and the M.S. and Ph.D. degrees in electrical engineering and computer sci-ence from the Massachusetts Institute of Technology (MIT), Cambridge, MA,in 2001 and 2005, respectively.

From 2000 to 2005, she was a Research Assistant at the Plasma Science andFusion Center and the Francis Bitter Magnet Laboratory, MIT. She was involvedin the design, development, testing, and analysis of a novel submillimeter wavesecond harmonic gyrotron oscillator, as well as other projects in the millimeterand submillimeter regime. Since 2005, she has been at the Naval Research Labo-ratory as an NRC postdoc in the Plasma Physics Division. Her research interestsinclude terahertz sources and their applications, such as enhanced nuclear mag-netic resonance spectroscopy via dynamic nuclear polarization.

Kan-Nian Hu received the B.A. and M.S. degreesin chemistry from National Taiwan University,Taipei, Taiwan, in 1996 and 1998, respectively. Heis currently working toward the Ph.D. degree inphysical chemistry at the Massachusetts Institute ofTechnology (MIT), Cambridge.

Since 2000, he has been a Research Assistant withthe Francis Bitter Magnet Laboratory, MIT, where hisresearch involves the development of DNP for sen-sitivity enhancement in NMR spectroscopy, in par-ticular designing polarizing agents for DNP at high

magnetic fields above 5 Tesla.

Kenneth E. Kreischer (M’88) received the B.S. degree in physics and the Ph.D.degree in nuclear engineering from the Massachusetts Institute of Technology,Cambridge, in 1976 and 1981, respectively.

He remained at MIT from 1981 to 2000, where he was a Principal ResearchScientist at the Plasma Science and Fusion Center, MIT. His main responsi-bility at MIT was overseeing the experimental research activities of the Wavesand Beam Division. A major area of research was the design and testing ofMW gyrotron oscillators operating at 100–300 GHz suitable for the heatingof fusion plasmas. Other responsibilities included the development of a highgradient accelerator operating at 17 GHz, an RF photocathode gun capable ofproducing a high brightness electron beam, and submillimeter instrumentationthat increases the sensitivity of EPR and NMR spectrometers used in chemistrymolecular research. He joined Northrop Grumman Corporation in 2000, and ispresently Director of the Vacuum Electronics Technology Research and Devel-opment Group. This group is responsible for developing high-power, broadbandmicrowave power modules that are used in ECM transmitters. They are also de-signing and testing novel THz technology that will be used as part of a highresolution imaging system being developed by DARPA. In addition to THz in-strumentation and broadband sources, his research interests also include highfrequency sources suitable for radar and communication, nonlethal active de-nial systems based on high-power microwaves, field emission electron sources,and agile beam steering.

Dr. Kreischer is presently a member of the American Physical Society andPhi Beta Kappa.

Melanie Rosay photograph and biography not available at the time of publica-tion.

Michael A. Shapiro (M’01) received the Ph.D. de-gree in radio physics from the University of Gorky,Gorky, Russia, in 1990.

In 1995, he joined the Plasma Science and FusionCenter, Massachusetts Institute of Technology(MIT), Cambridge, where he is currently Head ofthe Gyrotron Research Group. His research interestsinclude vacuum microwave electron devices, highpower gyrotrons, dynamic nuclear polarization spec-troscopy, high gradient linear accelerator structures,quasi-optical millimeter-wave components, and

photonic bandgap structures.

Jagadishwar R. Sirigiri (S’98–M’02) received theB.Tech. degree in electronics and communication en-gineering from the Institute of Technology, BanarasHindu University (BHU), India, in 1996, and theM.S. and Ph.D. degrees in electrical engineering andcomputer science from the Massachusetts Instituteof Technology (MIT), Cambridge, in 1999 and 2002,respectively.

From 1994 to 1996, he was associated with theCenter of Research in Microwave Tubes, BHU,where his research areas included TWTs and

gyro-TWTs. From 1996 to 1998, he was with Global Research and Develop-ment, Wipro Infotech, Ltd., where he worked in the multimedia research group.Since 2002, he has been a Postdoctoral Research Associate at the PlasmaScience and Fusion Center, MIT. He is involved in the design and developmentof novel high-power gyrotrons and gyrotron amplifiers at millimeter-wavefrequencies. His research interests include novel microwave sources andamplifiers in the millimeter and submillimeter regime such as the gyrotron,quasi-optical structures, and photonic band gap (PBG) structures and theirapplications in microwave vacuum electronics.

Richard J. Temkin (M’87–F’94) received the B.A.degree in physics from Harvard College, Cambridge,MA, and the Ph.D. degree in physics from the Mass-achusetts Institute of Technology (MIT), Cambridge.

From 1971 to 1974, he was a Postdoctoral Re-search Fellow in the Division of Engineering andApplied Physics, Harvard University. Since 1974, hehas been with MIT, first at the Francis Bitter NationalMagnet Laboratory and later at the Plasma Scienceand Fusion Center (PSFC) and the Department ofPhysics. He currently serves as a Senior Scientist

in the Physics Department, as Associate Director of the PSFC, and Head ofthe Waves and Beams Division, PSFC. His research interests include novelvacuum electron devices such as the gyrotron and free electron laser, advanced,high-gradient electron accelerators, quasi-optical waveguides and antennas atmillimeter wavelengths, plasma heating, and electron spin resonance spec-troscopy. He has been the author or coauthor of over 200 published journalarticles and book chapters and has been the editor of six books and conferenceproceedings.

Dr. Temkin is a Fellow of the American Physical Society and The Institute ofPhysics, London, U.K. He has been the recipient of the Kenneth J. Button Prizeand Medal of The Institute of Physics, London and the Robert L.Woods Awardof the Department of Defense for Excellence in Vacuum Electronics research.

Paul P. Woskov (S’74–M’76–SM’99) received thePh.D. degree in electrical engineering from the Rens-selaer Polytechnic Institute, Troy, NY, in 1976.

In 1976, he joined the Francis Bitter NationalMagnet Laboratory, Massachusetts Institute ofTechnology (MIT), Cambridge, where, since 1980he has been with the MIT Plasma Science andFusion Center. He is currently a Principal ResearchEngineer and Associate Division Head of the PlasmaTechnology Division. His principal interests includeplasma diagnostics, fusion energy, millimeter-wave

technologies, and environmental applications of plasmas and millimeter waves.Dr. Woskov is a member of the American Physical Society, the American

Chemical Society, and the American Association for the Advancement of Sci-ence.